Coastal Engineering Journal

With the focus on the characteristics of temporal and spatial fluctuations of the bottom-sediment properties and their vertical distributions around the mouth of the Tama River in Tokyo Bay, the properties of surface sediments, such as grain-size distributions and water contents, had been monitored by taking bottom-sediment core samples from several sampling stations around the river mouth with the intervals of 3 month for a period of five years from 2006 through 2010. Based on the monitored data, the impact of flood events which occurred during the survey period on bottom sediment properties and the recovery processes after the flood event were evaluated. This study clarified that seasonal fluctuations of surface bottom sediment qualities clearly appear on river mouth shoals and tidal wetlands and the impact of the river flood was relatively slight and the damages were restored over relatively short period of time after the flood event. On the other hand, the flood event remarkably caused rapid decline of the water content of bottom sediments at offshore of the river mouth, and it took about three years for their bottom sediments to return to its normal as before the flood event. This paper also discusses the recovery process of the sediment properties at the offshore region and shows that the long-term evolution of the sediment properties was caused by the constant settling of new deposition of suspended particles from the water column and accumulation rate was estimated at 2.5 cm per year.

Long period waterlevel variations, from 14 d to 1 yr, are examined for Hamanako, a tidal inlet on the Pacific coast of Honshu, Japan. Eleven years of records, prior to the regulation of the entrance to the inlet, are analyzed. At all the frequencies examined there were substantial fluctuations, some periodic and some irregular. Dominant features were abrupt changes in mean waterlevel and tidal range, and regular 14 d and 28 d cycles in most years. These cycles have been related to the role of the entrance constriction in generating spring-tidal pumping (STP). The rapid changes in tidal amplitude and mean waterlevel are explained in terms of changes in the hydraulic resistance of the entrance and have been correlated with outflows from the tidal lagoon caused by typhoons and catchment runoff. An annual cycle of amplitude 12.5 cm has been explained in terms of the annual cycle of the Kuroshio. Novel features of this study are the extensions to conventional time series analysis, and showing that the 14 d periodic fluctuations were caused by STP and that this process also contributed to the 28 d fluctuations. STP is invariably present in estuaries with a shallow inlet but is rarely identified; this paper shows methods to evaluate it and other long period components of the waterlevel record.

Suspended sediment transport plays an important role in the evolution of coastal environment. Hence, observation and modeling of suspended sediment concentration (SSC) is of immense interest to coastal oceanographers, engineers and coastal zone management authorities. The variation of SSC in an estuary is expected both in space and time scales governed by factors such as freshwater influx, tidal variation and basin geometry. The present study reports on development of SSC model for the tidal dominated Hooghly basin, a tributary of the Ganges River having its discharge in the Bay of Bengal, India. The Hooghly estuary experiences high rates of sedimentation related problems requiring periodic maintenance dredging at various locations in the basin. Being a tidal dominated estuary, varying loads of suspended sediments is distributed along various channels in the estuarine zone. The existing SSC models developed earlier for this region had inherent limitations that tend to over-predict computed SSC. These issues critically investigated in the present study thereby developing an upgraded SSC model. It includes better physical parametrizations compared to existing SSC models developed for this region. Efficacy and robustness of the developed SSC model substantiated with measurements conducted in the Hooghly basin. Validation of model computed SSC performed with all available data that comprises of 15 location specific observations of SSC. The validation results signify that upgraded SSC model performs reasonably well compared to field measurements.

In this study, a simplified multicomponent technique for modeling the 2D sediment transport in the vicinity of navigation channels to harbors is formulated. The main simplification assumes that longshore current is the only factor transporting sediments to the offshore navigation channel. The technique requires the application of three numerical models: NMLONG model (1D depth averaged finite difference wave model), RMA2 (2D depth averaged finite element hydrodynamic model) and SED2D (2D depth averaged finite element sediment transport model). RMA2 is forced at the inflow boundary using longshore velocity profiles generated by the incoming waves via the application of NMLONG model. Output of NMLONG provides also the boundary condition of SED2D which takes a velocity field input from RMA2. A variable manning coefficient is used inside RMA2 domain to account for wave roughness. Data collected in the period from 1989 to 1997 in the vicinity of the navigation channel of Damietta Harbor (Egypt) is used to calibrate the multicomponent technique. The results show that the technique explains well the observed spatial variation of bed change in the navigation channel.

The calibrated technique is then applied to study different solutions to reduce sedimentation in Damietta Harbor navigation channel. The results showed that the optimum solution found can significantly reduce the amount of sedimentation. For sedimentation caused by west-to-east longshore current, the reduction amount range is 62–74% (i.e. only 26–38% of the current sedimentation shall occur). For sedimentation caused by east-to-west longshore current, the reduction percentage reaches 60–70%.